Move TurboModuleRegistry (JS) to github

Summary:
Moved the JS wrapper function to github. To access a TurboModule from JS:

```
export interface Spec extends TurboModule {
  +func1: () => number,
}

const module = TurboModuleRegistry.get<Spec>('SampleTurboModule');
```

This assumes:
* the binding on the native side has been installed properly, i.e. `global.__turboModuleProxy` needs to be installed properly.
* the module `SampleTurboModule` is registered properly in native.

More instructions will be provided later.

Reviewed By: yungsters

Differential Revision: D13584561

fbshipit-source-id: 50d29d88787f8d9caa7a3ee0d54d378db866515c

Libraries/TurboModule/RCTExport.js

@@ -0,0 +1,35 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict
+ * @format
+ */
+
+'use strict';
+
+/**
+ * NOTE: This is React Native specific export type.
+ *
+ * RCTExport is an interface type that allows native code generation for React
+ * Native native modules. It exists as a hint to the codegen tool that any
+ * interface that extends it needs to be codegen'ed. Example usage:
+ *
+ *   export interface RCTFoobar extends RCTExport<void> {}
+ *
+ * Native definition for RCTFoobar will then be generated.
+ *
+ * The type param T is a placeholder for future codegen hinting, like versioning
+ * information, native base classes, etc. For now, simply use `void` type as
+ * there's nothing to give hint about.
+ */
+
+// eslint-disable-next-line no-unused-vars
+export interface RCTExport<T: void = void> {
+  +getConstants?: () => {};
+}
+
+// eslint-disable-next-line lint/react-native-modules
+export interface TurboModule extends RCTExport<void> {}

Libraries/TurboModule/TurboModuleRegistry.js

@@ -0,0 +1,35 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ * @format
+ */
+
+'use strict';
+
+import type {TurboModule} from 'RCTExport';
+import invariant from 'invariant';
+import {NativeModules} from 'react-native';
+
+// TODO
+function get<T: TurboModule>(name: string): ?T {
+  // Backward compatibility layer during migration.
+  if (NativeModules[name] != null) {
+    return ((NativeModules[name]: any): T);
+  }
+
+  const module: ?T = global.__turboModuleProxy(name);
+  return module;
+}
+
+function getEnforcing<T: TurboModule>(name: string): T {
+  const module = get(name);
+  invariant(module != null, `${name} is not available in this app.`);
+  return module;
+}
+
+export {get};
+export {getEnforcing};